Adding soft-start for LDO LT3080

QYQ on Oct 26, 2020

Hi,

I have a question about the below circuit of LT3080, what is the purpose of D1?

Is that for discharge C2 when the Vin disappeared?

Because D1 will affect the accuracy of Vout, I do not want to add D1。

Best Regards,

Leo

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ARad
Oct 28, 2020 +1 suggested
Thanks for the post. Yes, the diode D1 you show will affect accuracy. I show how much a silicon diode can leak in the image below. As far as I know, the purpose of D1 is to prevent SET from being held…
ARad
Nov 6, 2020 in reply to QYQ +1 verified
This post discusses the need to clamp OUT to SET - usually when a voltage is applied to SET - but your situation may apply as well. The post is titled "LT3083 output not zero when set pin grounded".…

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on Oct 27, 2020 4:11 PM over 1 year ago
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0 ARad

on Oct 28, 2020 3:59 PM over 1 year ago
Thanks for the post. Yes, the diode D1 you show will affect accuracy. I show how much a silicon diode can leak in the image below. As far as I know, the purpose of D1 is to prevent SET from being held up if IN=VCONTROL is forced low. Actually, if IN=VCONTROL is forced low, OUT will also be forced low because there is a parasitic diode from OUT to IN on the LT3080, similar to the parasitic diode I discuss in this post titled "Disable LT3083".
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0 QYQ on Oct 30, 2020 3:55 PM over 1 year ago in reply to ARad

Hi,

Very thank you for your reply.

ABS.max of SET pin voltage is just ±0.3V relative to OUT.

it seems that If I use a little big C2 for soft-start, that means it is a must to use D1 to protect SET pin when IN=VCONTROL is forced low. this will be a weak point.

Best regards,

Leo
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0 QYQ on Nov 6, 2020 7:33 AM over 1 year ago in reply to QYQ

Hi Arad,

Please let me ask one more question.

Because ABS.max of SET pin voltage is just ±0.3V relative to OUT, is it necessary to place a diode between SET pin and OUT pin?

Thanks,

Leo
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+1 ARad

on Nov 6, 2020 3:32 PM over 1 year ago in reply to QYQ
This post discusses the need to clamp OUT to SET - usually when a voltage is applied to SET - but your situation may apply as well. The post is titled "LT3083 output not zero when set pin grounded".

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